Spacecraft are often launched on a booster which places the spacecraft in an orbit which is lower than the desired orbit. In order for a spacecraft to achieve the desired orbit, it may be necessary to operate a velocity change (delta-V or .DELTA.V) thruster mounted on the spacecraft body.
When spin-stabilized spacecraft are accelerated by a velocity change thruster such as a liquid apogee thruster (LAE), the spinning spacecraft body provides gyroscopic stiffness which tends to reduce deviations of the longitudinal axis of the body from the desired pointing direction. In this context, the pointing direction is the direction in which the velocity change is desired. Three-axis stabilized spacecraft, however, do not rely on spin stabilization, and are stabilized by the operation of an attitude control system, which uses controllable torquers to rotate the body about the three axes of control. These axes are normally designated as the yaw, roll, and pitch axes.
When a spacecraft is fabricated, a great deal of attention is paid to aligning the thrust axes of the various thrusters relative to the center of mass of the spacecraft in a manner which results, when each thruster is fired, in the desired torque. In the case of attitude control thrusters, the desired torques are those around the three principal axes. In the case of the velocity change thruster, the desired torque is zero around each of the three body axes. That is, the velocity change thruster desirably imparts only a velocity change in the direction of its axis of thrust, and ideally does not impart a torque to the spacecraft body. In order to minimize the torques which the velocity change thruster imparts to the spacecraft body, its thrust axis is aligned as closely as possible with the center of mass of the spacecraft body.
In reality, it is not possible to determine the exact location of the center of mass of the spacecraft body, or to align the thrust axis of the velocity change thruster therewith. It is also difficult to determine the exact axis of thrust of the thruster. Consequently, operation of the velocity change thruster always results in some unwanted torques, which tend to rotate the spacecraft away from the desired pointing direction.
The attitude of a three-axis stabilized spacecraft is controlled by its attitude control system, as by operating momentum or reaction wheels, by magnetic torquing, or by operating attitude control thrusters. The disturbance torques occasioned by operation of the velocity change thruster are opposed by the torques produced by the attitude control system. However, due to the finite bandwidth of the attitude control system, there is some delay between the torque occasioned by the velocity change thruster and the opposing correction torque produced by the attitude control system. This delay results in a pointing error, which exists or continues until the attitude control system applies control to fully counteract the disturbance and realign the spacecraft with the desired pointing directions. During the interval between initiation of operation of the velocity S change thruster and the stabilization of the attitude control system, a component of velocity change occurs in an unwanted direction, which is not corrected by the attitude control system.
Improved spacecraft control systems are desired.